Passivated magneto-resistive bit structure

ABSTRACT

A passivated magneto-resistive bit structure is disclosed in which surfaces subjects to oxidation or corrosion are protected. In one embodiment, a bit structure is encapsulated by means of an etch stop barrier material. In another embodiment an etch stop barrier material protects the top of a bit structure and dielectric spacers protect the side walls.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 09/638,419, filed Aug. 14, 2000, now U.S. Pat. No. 6,392,922, entitled “PASSIVATED-MAGNETO-RESISTIVE BIT STRUCTURE AND PASSIVATION METHOD THEREFOR,” which is incorporated herein by reference.

This invention was made with Government support under Contract Number MDA972-98-C-0021 awarded by DARPA. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to magneto-resistive memories, and more particularly, to the passivation of magneto-resistive bit structures.

Typical magneto-resistive memories such as giant magneto-resistive random access memory cells (GMR MRAM's) use variations in the magnetization direction of a thin film of ferromagnetic material in a GMR stack can be referred to as a magneto-resistive bit. During a write operation, the magnetization direction of a selected bit is set by passing an appropriate current near the bit, often using a word line, digital line, or sense line. The current produces a magnetic field that sets the magnetization direction of at least one of the layers in the ferromagnetic film in a desired direction. The magnetization direction dictates the magneto-resistance of the film. During a subsequent read operation, the magneto-resistance of the film can be read by passing a sense current through the bit structure via a sense line or the like.

Some prior art magneto-resistive bit structures are shown and described in U.S. Pat. No. 4,731,757 to Daughton et al. and U.S. Pat. No. 4,780,848 to Daughton et al., both of which are assigned to the assignee of the present invention and both of which are incorporated herein by reference. Illustrative processes for forming such magnetic bit structures are shown and described in U.S. Pat. No. 5,496,759 to Yue et al., and U.S. Pat. No. 5,569,617 to Yeh et al., both of which are assigned to the assignee of the present invention and both of which are incorporated herein by reference.

Such magneto-resistive memories are often conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the bit structures and the memory operating circuitry on the monolithic integrated circuit. To provide a sense current through the bit structure, for example, the ends of the bit structure are typically connected to adjacent bit structures through a metal interconnect layer. The string of bit structures then forms a sense line, which is often controlled by operating circuitry located elsewhere on the monolithic integrated circuit.

A problem which arises as a result of the use of magneto-resistive memories is that conventional integrated circuit processes often cannot be used to form the contact holes or vias that are used to provide connections to the bit structure. For example, in a conventional integrated circuit process, vias are often formed by means of an etching process. First, a patterned photoresist layer, which defines the location and size of the vias, is provided over the integrated circuit. With the photoresist layer in place, vias are etched down to the bit structure. Once the vias are etched, a photoresist removal step typically is used to remove the photoresist layer.

In a typical integrated circuit back-end process the GMR bit ends are susceptible to damage by the corrosive chemicals used in the etching process. Furthermore, GMR bit ends may be exposed to a plasma environment with oxygen during removal of the photoresist in the oxygen asher process and are left unpassivated thereafter. Oxidation of the side walls of the bit ends can lead to significant degradation and adversely affect performance of the GMR MRAM's. In order to avoid potential disastrous consequences, oxygen plasma photoresist removal is not generally utilized as the post Permalloy via etch or subsequent M3 etch stages. Instead, various solvent photoresist strip process are utilized to remove the photoresist layers. Where a solvent or “wet” photoresist strip is used, it is necessary to choose the solvent with extreme care and to limit solvent use to mild solvents. In general, wet photoresist strips, although reducing the risk of oxidation, are prone to other defects and are not very production-worthy.

What would be desirable, therefore, is a magneto-resistive bit structure which is not subject to oxidation or corrosion by processing steps when forming vias. More specifically, it would be desirable to form a magneto-resistive bit structure without directly exposing the side walls of the bit ends to the potentially adverse effects of processes involved in via formation. This may allow more efficient and reliable back-end processing, which, in turn may reduce the defect density and increase the overall performance of devices incorporating magneto-resistive memories.

SUMMARY OF THE INVENTION

The present invention overcomes many of the disadvantages of the prior art by providing a magneto-resistive bit structure wherein the bits are protected from the potentially adverse effects of process steps that could damage the unprotected bit structure.

In one embodiment of the invention, magneto-resistive bits are encapsulated by means of an etch stop barrier material, such as CrSi, which is deposited as a barrier to cover both the top surface and side walls of a GMR stack. Encapsulation of GMR cells is highly desirable in the switching characteristics of the cells. The etch stop barrier material, preferably CrSi, provides a good barrier to oxygen and corrosive chemicals and thereby improves the robustness of GMR cells. It is selected to have a bulk resistivity which is low enough to allow sense line contact and high enough so that shunting will be negligible.

In another embodiment of the present invention, an etch stop barrier layer, for example, a layer of CrSi protects the top of the GMR stack while the side walls are protected by means of a dielectric extending along the edges of the GMR stack. For example, a SiN spacer can be formed by means of a controlled Permalloy via etch.

Metals deposited upon the passivated bit structure contact the bit ends only from the top through the etch stop layer, while the side walls of the bit ends are insulated from the metal by the dielectric spacer.

Passivation of magneto-resistive bit structures by the methods of this invention significantly improves the repeatability of GMR cells and makes the GMR backend process more production-worthy.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects of the present invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designated like parts throughout the figures thereof and wherein:

FIG. 1 is a plan view, partly in phantom, of one illustrative embodiment of the passivated magneto-resistive bit structures of the present invention;

FIG. 2 is a plan view showing sense line metal contacts on top of the passivated magneto-resistive bit structure shown in FIG. 1;

FIGS. 3 to 10 are cross-sectional views showing certain of the steps of a process for forming a passivated magneto-resistive bit structure according to one embodiment of the present invention; and

FIGS. 11 to 15 are cross-sectional views showing certain of the steps of a process for forming the passivated magneto-resistive bit structure according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a plan view of an illustrative embodiment of the present invention. An array of passivated magneto-resistive bit structures are shown generally at 2. A passivating material 4 is deposited on top of magneto-resistive bit 6, shown in phantom. FIG. 2 is similar to FIG. 1 with sense line metal contacts 8 deposited on top of passivating material 4, which is at least partially conductive.

A process for forming a passivated magneto-resistive bit structure according to a preferred embodiment of the present invention will now be illustrated by reference to FIGS. 3 to 10.

Referring to FIG. 3, a GMR stack 32 is shown deposited upon a SiN substrate 30, which may be a 500 Å SiN layer. Alternatively, substrate 30 may be a monolithic integrated circuit or an integrated circuit separated from GMR stack layer 32 by a dielectric layer. A SiN layer 34 is deposited upon GMR stack 32. SiN layer 34 may be an 800 Å SiN layer.

In the next step of the process, an ion mill mask is built upon GMR stack 32 using the SiN layer 34. FIG. 4 shows ion mill mask 34 a formed from SiN layer 34. Part of SiN layer 34 is etched away by reactive ion etching.

In the next step of the process, ion milling is used to pattern GMR stack 32. FIG. 5 shows a patterned GMR stack 32 a resulting from this step, in which ion mill mask 34 a is removed as well as part of GMR stack layer 32.

The next step in the overall process is the deposition of a barrier layer, which, after patterning, serves as the encapsulant in this embodiment of the present invention. As a barrier layer any etch stop material can be used which has a bulk resistivity low enough to allow sense line contact and high enough so that shunting will be negligible. CrSi is preferred. If desired, a thin Ta or TaN diffusion barrier can be deposited between the above-described barrier layer and the GMR stack to prevent intermixing. FIG. 6 shows a barrier layer 36, made up of a 300 Å CrSi layer on a 100 Å Ta layer, deposited upon patterned GMR stack 32 a and SiN substrate 30.

Following the Ta/CrSi barrier layer deposition a dielectric layer is deposited upon barrier layer 36. Any dielectric can be used. SiO₂ or SiN is preferred. FIG. 7 shows a 1000 Å SiO₂ layer 38 deposited upon Ta/CrSi barrier layer 36.

Following deposition of SiO₂ layer 38, in the next step, shown in FIG. 8, a part of dielectric layer 38 is removed by reactive ion etch to expose the unwanted portions of Ta/CrSi barrier layer 36, resulting in patterned dielectric layer 38 a.

With the unwanted portions of the Ta/CrSi barrier layer 36 exposed, ion milling is utilized to remove the unwanted portions of the Ta/CrSi barrier layer 36. FIG. 9 shows pattern Ta/CrSi barrier layer 36 a resulting from this step as well as a residual SiO₂ layer 38 b. Patterned SiO₂ layer 38 a in FIG. 8 has been reduced in thickness as a result of the ion milling step to result in residual SiO₂ layer 38 b having a thickness of 400 Å.

Finally, as shown in FIG. 10 residual SiO₂ layer 38 b from FIG. 9 is removed by reactive ion etch, resulting in GMR stack 32 a being fully encapsulated by Ta/CrSi barrier 36 a. Thus, a passivated magneto-resistive bit structure is provided by encapsulation of the GMR stack by means of a Ta/CrSi barrier. Both the tops and the side walls of the GMR bits are protected by passivation in the manner described above. The side walls of the GMR bits are not exposed either during Permalloy via etch or thereafter. Therefore, oxygen asher photoresist removal and conventional wet chemical strips can be used without damage to the passivated GMR bits.

An alternative embodiment of the present invention will now be illustrated in reference to FIGS. 11 to 15. Referring to FIG. 11, an underlayer 40, which may be a monolithic integrated circuit, serves as a substrate for a GMR stack 42. Underlayer 40 preferably includes a dielectric layer separating the GMR stack from an integrated circuit. The dielectric layer may be, for example, a 500 Å seed SiN layer, not shown. A CrSi barrier layer 44 is deposited on top of GMR stack 42. A SiN ion mill mask 46 is fabricated in a known manner on top of CrSi layer 44. For example, a relatively thick SiN layer can be deposited upon CrSi layer 44 and patterned using a photoresist and reactive ion etch. Layer 44 may, for example, be an 800 Å thick CrSi layer. CrSi is preferred because, in addition to being a good oxygen barrier and etch stop, CrSi has a bulk resistivity low enough to allow sense line contact and high enough to minimize shunting. Ion mill mask 46 may be a 2000 Å SiN layer. Other suitable dielectric materials such as SiO₂ can be used, if desired. SiN is preferred because, in addition to being an excellent dielectric, it provides an efficient barrier against oxidation and protects GMR bits from other damaging materials as well.

The next steps in the process is illustrated in FIG. 12, wherein the results of ion milling are shown. It can be seen that portions of CrSi layer 44 and portions of GMR stack 42 have been removed, resulting in patterned GMR stack 42 a and patterned CrSi barrier layer 44 a. The ion mill mask is then separately removed. As shown, CrSi layer 44 a has been reduced from a thickness of 800 Å to a remaining thickness of 700 Å.

The next step in the process is deposition of a layer of a dielectric material, such as a 2000 Å layer of SiN or SiO₂, followed by planarization, such as by using high angle ion milling or chemical mechanical polishing (CMP). FIG. 13 shows a planarized layer of SiN 48 deposited upon the structure of FIG. 12.

The next process step is a controlled Permalloy via etch, the results of which are illustrated in FIG. 14. In this step most of the SiN layer 48 is removed, leaving naturally created SiN passivation spacers 48 a along the side walls 42 b of patterned GMR stack 42 a and providing a passivated magneto-resistive bit structure. Spacers 48 a are shown abutting the side walls 42 b of patterned GMR stack 42 a and the side walls 44 b of patterned CrSi layer 44 a. Spacers 48 a are shown having side walls 48 b, which are spaced out laterally from the side walls 42 b of GMR stack 42 a and side walls 44 b of patterned CrSi layer 44 a.

In FIG. 15 a contact metal 50 is deposited over the passivated magneto-resistive bit structure of FIG. 14. Contact metal 50 does not contact GMR stack 42 a directly at any point, since the side walls 42 b of GMR stack 42 a are protected by spacers 48 a and the top of GMR stack 42 a is protected by CrSi layer 44 a. Indirect contact between GMR stack 42 a and contact meal 50 is possible only at the top of GMR stack 42 a through CrSi layer 44 a, which conducts electric current between contact metal 50 and GMR stack 42 a. The SiN spacers provide an excellent barrier against oxidation and contact by corrosive chemicals.

Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantageous of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. 

We claim:
 1. A passivated magneto-resistive bit structure, comprising: a magneto-resistive bit having a top surface and side walls; a conductive etch stop barrier layer encapsulating the top surface and sidewalls side walls of said bit; and a metal contact provided upon substantially all of a top surface of said etch stop barrier layer including at least a portion of the etch stop barrier layer encapsulating the side walls of the magneto-resistive bit.
 2. The bit structure of claim 1, further comprising a diffusion barrier between said etch stop barrier layer and said bit.
 3. The bit structure of claim 2, wherein said diffusion barrier comprises a material selected from the group consisting of Ta and TaN. 